1. Field of the Invention
The present invention relates to a diaphragm of an electric sound converter which converts vibration such as sound into an electrical signal, and relates particularly to a diaphragm in which there is a center dome portion reinforced while suppressing variations in the sensitivity and frequency response to the minimum, and a method of manufacturing the diaphragm.
2. Description of the Related Art
In a diaphragm in dynamic microphones and headphones, for example, in order to expand the low-pass reproduction limit, there is a method of forming a film of a sub-dome portion of the diaphragm such that the film thickness is small and reducing a resonance frequency. When this method is executed, a film such as polyester is placed on a forming die to be heated and processed, and a thin diaphragm is formed.
However, in the above case, not only the thickness of the sub-dome portion is reduced, but also the thickness of the center dome portion is reduced, and therefore, there is a problem that the mechanical strength of the center dome portion is reduced, and a high-pass reproduction limit is reduced, so that anomalous resonance easily occurs.
In order to solve the above problem, the center dome portion has been reinforced by a method of applying a reinforcing film having the same shape as that of the center dome portion to the center dome portion.
However, in the method of applying the reinforcing film to the center dome portion, when materials of the film and an adhesive are different from a material of the center dome portion, their thermal expansion coefficients are different from each other, and therefore there is a problem that a thermal deformation due to a temperature change occurs as in bimetal. Further, there is a problem that an individual difference is generated due to the thermal deformation and the frequency response and the sensitivity vary.
In order to solve the above problem, the present applicant has been proposed a diaphragm disclosed in Japanese Patent No. 3049570. In this diaphragm, a center dome portion is made of the same material as the diaphragm, and a reinforcing film formed to have the same shape as that of the center dome portion is applied with a hot-melt adhesive of the same nature as that of the diaphragm.
When the above diaphragm is manufactured, as shown in FIG. 10, a reinforcing film 51 having a diameter substantially equal to a diameter of a center dome is placed above a forming die 50 and then covered from the above with a base film 52, and a pressure pot (not shown) is moved toward the forming die, whereby the diaphragm is formed.
At this time, the reinforcing film 51 is also applied to the film with a hot-melt adhesive 53. At the same time, an organic solvent of the hot-melt adhesive 53 is vaporized by heat of the forming die 50, and the hot-melt adhesive 53 is cured. Consequently, a center dome 55 applied with the reinforcing film 51 at the central portion and a sub-dome 56 are formed as shown in FIG. 11.
According to the above constitution, since the base film 52, the reinforcing film 51, and the hot-melt adhesive 53 are formed of homogeneous materials, the variations in the sensitivity and the frequency response due to a temperature change can be suppressed to the minimum.
However, in the constitution disclosed in the Japanese Patent No. 3049570, when the hot-melt adhesive 53 is melted at the formation of the diaphragm, the hot-melt adhesive 53 is less likely to be spread in a horizontal direction because of the high viscosity, and a thickness unevenness of an adhesive layer of the center dome 55 may occur.
When the thickness unevenness of the adhesive layer of the center dome 55 occurs, there is a problem that an individual difference is generated in sound quality due to the variations in the sensitivity and the frequency response.